1. Field of the Invention
The present invention relates to an apparatus for examining a biological object by using radiation in which angiography can be effected in synchronism with the cardiac beat waves of the biological object.
2. Description of the Prior Art
In the conventional radiation diagnostic apparatus, for example, in the X-ray diagnostic apparatuses disclosed in U.S. Pat. Nos. 4,204,225 and 4,204,226, etc., the timing of the angiography of the circulatory organs, such as the blood vessels and the heart, are determined on the basis of predetermined times. That is, to obtain the mask images for a biological object (hereinbelow, simply referred to as an object) before an X-ray contrast medium reaches a predetermined portion of the object, e.g., a patient on the basis of the instant (T.sub.0) at which the injection or administration of the X-ray constrast medium commences, the X-ray is projected, or irradiated for (S.sub.1) seconds immediately after (T.sub.1) seconds have passed from the above instant (T.sub.0). Subsequently the X-ray is again projected for (S.sub.2) seconds immediately after (T.sub.2) seconds have passed from the instant (T.sub.0), thereby obtaining the contrast images of the object when the X-ray contrast medium reached the predetermined portion. In the case of using the pulsatory X-ray as the X-ray to be projected, pulse repetition rates PD.sub.1 and PD.sub.2 and pulse numbers PN.sub.1 and PN.sub.2 are substituted for the above projection times S.sub.1 and S.sub.2. These new projection times S.sub.1 and S.sub.2 are represented by the following equations. EQU S.sub.1 =PD.sub.1 .times.PN.sub.1 EQU S.sub.2 =PD.sub.2 .times.PN.sub.2
The mask images are digitally subtracted from the contrast images obtained in this way, so that the subtraction images with respect to only the diagnostic portion, e.g., blood vessels into which the X-ray contrast medium has been injected, are obtained and displayed.
On the other hand, the time period when the blood circulates, i.e., the time period when the X-ray contrast medium flows, does not depend on the above-described time, for example, the projection time, but depends upon the beat numbers of the patient. However, in the conventional angiographic apparatus, the timings are set entirely regardless of the blood circulating time depending upon the beat numbers of the biological object. Therefore, in the conventional X-ray diagnostic apparatus, the timings of X-ray projection are usually shifted with respect to the distribution of the X-ray contrast medium so that this causes a problem such that the contrast images are not collected at the moment the X-ray contrast medium appears with the highest concentration in the portions to be diagnosed. In addition, if the X-ray is continuously projected for a long time to solve this problem, there will also be a drawback of exposing the object to an excessive amount of X-rays during examination. Consequently, this also causes a danger of an increase in exposure dosage. On the other hand, if the digital subtraction is effected between the images obtained at different phases of the cardiac beat according to the timing setting irrespective of the beat numbers, there occurs an artifact in the subtraction images due to phase deviation of the cardiac beat. As a result, this artifact can make accurate diagnosis difficult.
The present invention is made in consideration of such circumstances and intends to provide an apparatus for examining a biological object by using radiation, in which by projecting the radiation and by collecting image data at timings based on the circulation rate of the blood, i.e., in synchronism with the cardiac beat or, heart beat, the exposure dosage to the object can be reduced as small as possible, and at the same time the diagnostic iamges with high picture quality can be displayed, thereby enabling the accurate diagnosis to be performed.